1. Field of the Invention
The present invention relates to partially composite lightweight metal products that are used where there are demands for superior abrasion resistance and strength as such as a break rotor or the like for vehicles, and to a porous preform which is used to produce the partially composite lightweight metal products.
2. Description of Related Art
In regards to components, such as a break rotor or the like for vehicles, which call for high wear resistance, casting iron has been typically used. The adaptability of various materials has been tried from the view point of improving the braking ability of a vehicle through an improved cooling down performance and an heightened friction coefficient of the break rotor together with improving the running stability through a reduction in non-suspended weight, i e. the weight of components such as wheels beneath chassis springs, by using lightweight products.
As one of these technologies, an aluminum alloy ingot in which SiC particles are dispersed is used to make products such as brake rotors by sand mold casting or metal mold casting. Such a break rotor has been employed in electric cars, sports cars and the like. However, there is the following sort of problems wherein room for further improvement remains even if applying the aluminum alloy ingot to the brake rotor. In this sort of technology, since SiC, which is a ceramic, is at a less choice of particle volumes and sizes, there is a limit to increasing the coefficient of friction of the brake rotor. In addition, in this sort of technology, it is necessary to use a base material that has favorable affinity with SiC. While an aluminum alloy that corresponds to AC4C is employed as the base material from this point of view, the utilization of an aluminum alloy as the base material causes the problem in that the useful limiting temperature drops. In particular, the break rotor suffers about 450.degree. C. during braking and wears soon which is the problem to be solved. Moreover, since the product which has high wear resistance is hardly workable because it contains reinforcements all over, not only the cost of material of the product is high, but the cost of machining the product is also high.
In recent years, there has been proposed, as one of methods of producing partially composite products of which only part (for example, a sliding surface section) of a product is made to as a composite part for reinforcement, to prepare a porous preform having specified shape of continuous pores first, impregnate the porous preform with a molten lightweight metal and pressurize the molten lightweight metal to fill it in the pores of the porous preform. There have been various proposals of porous preforms used in the method of producing partially composite products and a method of producing the porous preform.
One of such a porous preform, that is known from, for example, Japanese Laid-Open Patent Application No. 7-108370, contains TiO.sub.2 and inorganic fibers. Another porous preform, that is known from, for example, Japanese Laid-Open Patent Application No. 6-182524, contains aluminum borate whiskers, sodium silicate, and silicon carbide.
The prior art porous preform is hardly adaptable to one which is applied to products having a demand for superior wear resistance and high strength such as a break rotor and expected to be improved at characteristics as matters stand. That is, even though applying the prior art porous preform to partially composite lightweight metal products such as a break rotor, such a partially composite lightweight metal product suffers irregularities generating on its sliding surface relatively soon, which leads to a cause of generating noises and vibrations or shudders. In the case where a porous preform such as described above is used to manufacture partially composite lightweight metal products, it is typical to pour a molten lightweight metal such as a molten aluminum alloy in a cavity of a metal mold with the porous preform set therein and fill in the pores of the porous preform so as thereby to tuck up and cover the porous preform with the molten lightweight metal. However, the prior art preform porous bodies are intended to be applied to manufacturing relatively small products, and, even if attempted to manufacture partially composite lightweight metal products that are applied to relatively large sizes of parts such as a break rotor, they suffer such a problem that the product is apt to form non-composite portions easily due to insufficient impregnation of the molten lightweight metal in the pores, as a result of which the final product, i.e. the brake rotor, experiences aggravation of wearing characteristics and strength.